The reason you see a lot of 6GB kits is that the 435T supports tri-channel memory, not just dual-channel.
To take advantage of the tri-channel you have to install in sets of 3 - you have 6 slots and 3 channels. Your current 8GB setup is probably in dual channel - I'm guessing 4 2GB modules, but that's just a guess. However, in general, mismatched single modules in one channel are single, matched pairs are dual-channel, matched triplets are tri-channel.
That may not be a big deal - like dual-channel, tri-channel is only an advantage if memory bandwidth is bottlenecking your computer. At least historically, more memory capacity is better than more bandwidth (or a higher access speed), but with the Core i7 technology being so new I could be wrong.
I think of memory "bandwidth" as being much like highways. A highway with 3 lanes both ways (total, 6 lanes, as in tri-channel) can theoretically carry more traffic more efficiently than 3 separate 2-lane highways (single channel), or a 2 lane highway + a service road (dual channel + a single mixed in). But, if the only traffic on the highway is a few chicken trucks and the odd Buick or Honda, they'll all get there at the same time - bandwidth doesn't really matter. But if you've got a highway full of Ferrari-type programs that use tons of memory and use your processor at full tilt, that's another story entirely - at that point, bandwidth starts to count. 
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Another thing - whether you need more memory or not depends on how much you actually use NOW. If you're already using most of the current 8GB (check your Task Manager to see), then more is good (and tri-channel is better), but if you're not anywhere near using your current 8GB, more memory will just sit there and go unused.
As far as Dell vs. Crucial: Neither company manufactures RAM, they just sell it (though Micron, Crucial's parent company, does make some RAM, they mostly sell other brands). My personal opinion is to go with Crucial, for the simple reason that memory is their specialty, is usually cheaper, it's guaranteed compatible, and I suspect their customer support for returns, exchanges, etc. is a lot less hassle.
'Tis a poor carpenter that blames his tools.